Ion implantation is a process for depositing chemical species into a substrate by direct bombardment of the substrate with high-energy ions. In semiconductor fabrication, ion implanters are used primarily for doping processes that alter the type and level of conductivity of target materials. A precise doping profile in an integrated circuit (IC) substrate and its thin-film structure is often crucial for proper IC performance. To achieve a desired doping profile, one or more ion species may be implanted in different doses and at different energies.
Recently, carbon and oxygen ion implants have become more prevalent in the manufacture of IC devices. Such implants provide improved transistor performance.
Carbon and oxygen ions are typically generated in a plasma-based ion source. FIG. 1 shows a typical ion source 100 in an ion implanter. The ion source 100 comprises an ion source chamber 102 with conductive chamber walls 114. At one end of the ion source chamber 102 there is a cathode 106 having a tungsten filament 104 located therein. The tungsten filament 104 is coupled to a first power supply 108 capable of supplying a high current. The high current may heat the tungsten filament 104 to cause thermionic emission of electrons. A second power supply 110 may bias the cathode 106 at a much higher potential than the tungsten filament 104 to cause the emitted electrons to accelerate to the cathode and so heat up the cathode 106. The heated cathode 106 may then emit electrons into the ion source chamber 102. A third power supply 112 may bias the chamber walls 114 with respect to the cathode 106 so that the electrons are accelerated at a high energy into the arc chamber. A source magnet (not shown) may create a magnetic field B inside the ion source chamber 102 to confine the energetic electrons, and a repeller 116 at the other end of the ion source chamber 102 may be biased at a same or similar potential as the cathode 106 to repel the energetic electrons. A gas source 118 may supply a reactive species (e.g., carbon dioxide, carbon monoxide, or oxygen, or a mixture of these gases) into the ion source chamber 102. The gas source 118 is not limited to gas bottles or reservoirs, but may comprise, for example, an oven that heats up a substance to produce a desired gas. For implantation of carbon or oxygen ions, carbon or oxygen containing reactive species are required. The energetic electrons may interact with the reactive species to produce a plasma 10. An extraction electrode (not shown) may then extract ions 12 from the plasma 10 for use in the ion implanter.
Existing methods for generating carbon or oxygen ions are problematic. One significant problem is a limited lifetime of ion sources running carbon or oxygen containing reactive species. Reaction by-products can accumulate inside an ion source chamber within a few hours, reducing ion output or causing the ion output to become unstable. Replacement or maintenance of an ion source chamber results in an extended downtime for an ion implanter. In addition, carbon or oxygen ion generation in an ion source chamber tend to cause a negative impact on subsequent ion generation processes carried out in that ion source chamber. For example, in one ion source chamber, a ten-hour carbon run can lead to a 50% reduction in productivity of boron ions and a 10% reduction in productivity of phosphorous ions. It is believed that carbon or oxygen ion generation has a “poisoning” effect on interior walls of an ion source chamber. The “poisoning” effect can adversely change surface chemistry for subsequent reactions in the ion source chamber. For example, fractionation rate for certain reactive species may be reduced significantly.
In view of the foregoing, it would be desirable to provide a technique for improving ion implanter productivity which overcomes the above-described inadequacies and shortcomings.